The change removes all direct calls to mxml from the modules that need an XML
parser (lastfm.c, pipe.c, rssscanner.c and httpd_rsp.c).
Even with the help of mxml, reading XML is hard, so a layer is added which
helps deal with stuff like whitespace and CDATA. This should make OwnTone more
resilient to any XML variations it might receive.
The changes fixes issue #1677.
